Aluminum alloy

ABSTRACT

A CASTABLE, AGE-HARDENABLE, ALUMINUM-BASE ALLOY CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, 3.0 TO 4.5 MAGNESIUM, 0.6 TO 1.8 SILICON, 0.4 TO 1.5 SILVER, 3 TO 0.5 ZIRCONIUM, AND THE BALANCE ALUMINUM.

May 30, 1972 H D BEWLEY 3,666,45

ALUMINUM ALLOY Filed Aug. l5, 1970 3 Sheets-Sheet 1 A om imm.

.mm1 .m2 C. oz mm OOO? Gow Op INVENTOR. Henry Dale Bew/ey A TTORNEY.

a ssauavn May 30, W72 H. BEWLEY 3,666,45

ALUMINUM ALLOY )1H SSBNGHVH INVENTOR. Henry Dale Bew/ey ATTORNEY.

May 30, 1972 H. D. BEWLr-:Y 3,666,45

ALUMNUM ALLOY Filed Aug. l5, 1970 3 Sheets-Sheet 5 ALL ALLOYS AGED T0FULL HARDNESS PROPERTIES AS A FUNCTION OF COMPOSITION CYCLES FATI GU E(IScI OOOI) SSEIIlS INVENTOR. Henry Dale Bew/ey BY AT TURNE Y.

United States Patent Olice 3,666,451 ALUMINUM ALLOY Henry Dale Bewley,Paducah, Ky., assigner to the United States of America as represented bythe United States Atomic Energy Commission Filed Aug. 13, 1970, Ser. No.63,457 Int. Cl. C22c 21/02 U.S. Cl. 75-147 2 Claims ABSTRACT OF THEDISCLOSURE A castable, age-hardenable, aluminum-base alloy consistingessentially of, in weight percent, 3.0 to 4.5 magnesium, 0.6 to 1.8silicon, 0.4 to 1.5 silver, to 0.5 Zirconium, and the balance aluminum.

BACKGROUND OF THE INVENTION The invention described herein was made inthe course of, or under, a contract with the United States Atomic EnergyCommission. The present invention relates to an improved aluminum-basealloy. More particularly, it relates to a castable, corrosion-resistant,age-hardenable aluminum-base alloy having improved fatigue properties.

The basic process for the separation of the isotopes of uranium bygaseous diffusion centers around the pumping of uranium hexafluoridethrough a barrier and maintaining a proper pressure differential acrossthe barrier. This is accomplished with axial-flow compressors driven byan electric motor. The axial-How compressor consists mainly of a rotor,stator, blades, and casing. The rotor is a cylindrical drum with a shaftalong its axis, with several rows of blades attached around theperiphery of the rotor extending transversely to the rotor axis. Thestator is generally cone shaped and sits just outside the tips of therotor blades defining an annular opening between rotor and stator fromthe suction end to the discharge end. Blades similar to the rotor bladesare positioned in rows inside the stator. As the rotor turns, the gas tobe cornpressed is drawn from the suction end, with each row of bladesbuilding up the gas pressure as the gas flows from suction to dischargeend. Because of the combination of corrosion resistance, castability,and high strength-toweight ratio, aluminum alloys have been used as thematerial of choice for compressor components, especially compressorblades. One of the best alloys used for this purpose is identifiedherein as the reference 214X having a composition consisting essentiallyof, in weight percent, 3 to 4.5 magnesium, 0.6 to 1.8 silicon, and thebalance aluminum. This alloy is readily castable into compressor blades,exhibits moderate tensile properties (less than 30,000 p.s.i. ultimatetensile strength) and fairly high fatigue properties (about 13,000p.s.i. fatigue limit). As compressors are driven to higher power levels,more stringent demands are placed on compressor components, especiallycompressor blading. Compressor tests at high power levels show that theendurance limit or fatigue strength of the 214X alloy is exceededresulting, in turn, in extensive deblading and damage to othercompressor components.

SUMMARY OF TI-Il-I INVENTION It is, accordingly, the main object of thisinvention to provide a new and improved alloy having all of thedesirable properties of the aforementioned reference 214X alloy, butwith a heightened level of mechanical properties, especially fatiguestrength.

This invention is based on the discovery that small additions of silverand silver with zirconium within specied limits lead to a hard, strongalloy capable, with appropriate 3,666,451 Patented May 30, 1972 ertiesof the 214X alloy. Silver in this range of concentration has been foundeffective to modify the aging process in a manner which involvesinteraction with diffusioncontrolling vacancies. Both vacancy and solutediffusion to sinks are restrained by the presence of silver in theconcentrations used in this invention. Electron microscopy studies haveshown that silver in effective amounts changes the 2l4X-type alloy byproducing a tine, evenly dispersed precipitated phase as opposed to asegregated structure where any precipitated phase is bunched orassociated mainly with grain boundaries. Thus, the addition of silverwithin prescribed limits results in an appreciable increase in hardnessand tensile strength relative to the reference 214X alloy. Most of theimprovement in hardness and tensile strength is achieved by additions ofsilver in the range 0.4 to 1.0 silver. Additions of silver up to 1.5percent result in slight additional improvement and must be balancedagainst the cost of the additional silver.

When additions of zirconium are combined with silver, the fatiguestrength or endurance limit of the resultant alloy is far beyond thatknown for any similar aluminummagnesium-silicon alloy as represented bythe reference 214X system. Addition of zirconium up to 0.5 percentcombined with the aforementioned silver addition leads to someimprovement in hardness and tensile strength and, most importantly, to adramatic increase in endurance limit or fatigue strength. Concentrationsbeyond 0.5 percent tend toward development of a brittle secondary phase.

The drawings of FIGS. 1-3 are graphs in which FIG. l shows the effect ofsilver and zirconium alloying additions on the age hardening response at350 F.

FIG. 2 shows the eiect of silver on age hardening induced at 350 C.relative to the reference 214X alloy.

FIG. 3 shows the improvement in fatigue properties obtained by silverand zirconium additions in an alloy aged to full hardness.

Improved structural stability of the new alloy of this invention isbrought about by a combination of compositional as well as structuralmodification. This means that the addition of silver and/or zirconiumalone to the reference alloy will not, of itself, lead to the improvedresult. It must, in addition, undergo an aging process, by which ismeant that the composition-modified alloy must be heated to and held ata temperature which leads to the development of a secondary precipitatedphase as characterized by electron microscopy. The development of thisage-hardened condition is achieved by maintaining the silverorsilver-and-zirconium-modied alloy at a temperature in the range 300-350"F. for a period of time suicient to produce a desired maximum hardness.The development of a maximum age-hardening condition does not require aprior solution treatment but rather can be obtained from as-castmaterial. The alloy is relatively insensitive to the cooling rate fromthe melt and both diecast and permanent mold-cast parts respond well tothe aging treatment. The age-hardening or precipitate-inducingtemperature is 30D-350 F. Lower temperatures may be used, but thedevelopment of a maximum hardness level takes an inordinately long time.Higher temperatures than 350 E. up to as much as 500 F. can be used, butare not desirable because maximum hardness levels and associatedstrength properties are reduced.

Alloys within the scope of this invention can be melted and held instandard cast iron pots. The silver can be added in the metallic formand dissolution of even relatively large ingots is rapid. Experience hasshown that losses of silver from the melt are negligible even after aremelt operation. The zirconium can be added either in the form of azirconium containing ux or as a standard high zirconium aluminum basehardener. Additions by using the hardener are preferred. Zirconium meltlosses may occur; so periodic compositional checks and new zirconiumadditions are needed to insure maintenance of the proper level ofzirconium. Small permanent mold castings containing varying amounts ofsilver and zirconium to modify the reference 2l4X composition wereproduced from each resulting alloy to provide age-hardening, tensile,and fatigue test bars.

Age-hardening eiect Age-hardening tests were conducted on various castspecimens to test the effect of composition and prior fabricationhistory on the maximum achievable Rockwell Hardness. The elect ofalloying additions on the agehardening response of as-cast 214Xreference alloy at 350 yF. is shown in FIG. 1, which is a Plot ofhardness as a function of aging time at 350 F. It is seen from FIG. 1that appreciable increments of hardness can be imparted to the reference214X alloy by the addition of 0.9 percent silver and that furtherincrements of hardness can be imparted by including small amounts ofzirconium.

Although, as stated previously, the silver modified alloys do notrequire a solution treatment to respond to aging, a comparison of themodified alloys with 214X after such a treatment (8 hours at 960 F.followed by a water quench) reveals that the addition of silver willeffect the age hardening response. Aging tests conducted on a 214K and214X plus Ag alloy after a solution treatment have showed thesolution-treated hardness of all the alloys to be somewhat below theiras-cast values and that the 214X alloy without silver addition exhibitedlittle response to an aging treatment. However, the 214X plus 0.9 Agalloy responded to reach a maximum hardness of '73 Rock-well Hardness inonly l5 hours; the 2l4X plus 0.4 Ag alloy was intermediate in itsresponse. The agehardening behavior of solution-treated alloys is shownin FIG. 2.

It should be noted that the maximum hardness achieved at theage-hardening temperature will be maintained in the alloy so long as itis operated at temperatures below that used to introduce theage-hardening effect. Thus, a plot of hardness-versus-temperature at anytemperature below about 50 F. of the age-hardening temperature will showa constant hardness over an extended time.

Tensile testing- Tensile tests were conducted at room temperature on thealloys of this invention in both the as-cast and fullhard (as-cast andaged to full hardness at 350 F.) conditions. The results are shown inTable I below.

TAB LE I TS, YS, Elon gation, Alloy K s i. K s.i Percent As-east 2l 22.l i7. 1 7. 214X -l- 0. 9 Ag 29. 8 22. 4 3 214X+0.9 Ag+0.2 Zr 31.8 .0 3

.As-east plus aged to full hardness at 350 F.

28. 7 27. 5 1 30. 8 29. 0 l 44. 9 43. 4 2 39. (i 3S. 1 2

It will be seen that the yield strengths of the as-cast alloys weresimilar, but the chemically modified alloys exhibited higher ultimatetensile strength and greater elongation. After aging to full hardness,the silver-modified alloys exhibited vastly improved yield and ultimatetensile strength. The highest strength -was associated with the 214Kplus 0.9 Ag alloy. Further improvement in tensile properties may beachieved with silver additions up to 1.5 percent but the improvementsare slight and may not be economically justified.

Fatigue testing TABLE II Temperature Endurance Alloy condition limit,p.s.i

214K As-castI 13, 000 214K +09 Ag A1 10,000 214K plus 0.9 Ag B 2 14, 000214K plus 0.9 Ag plus 0.2 Zr A 12, 000 214K plus 0.9 Agplus 0.2Zr B10,000

I .as-cast plus aged to full hardness at 350 F. 2 As-cast plus aged (236hours at 350 F.)

The data in Table Il demonstrate the remarkable irnprovement inendurance limit achieved by chemical and physical modification of thebase of the reference 214X alloy, namely, by the addition of silver andzirconium combined with an aging treatment at temperature to produce afinely dispersed array of small age-hardening particles. As Table IIshows, the endurance limit of silvermodiiied alloys subjected to anaging treatment at 350 F. was raised to 14,000 p.s.i. as compared to asilvermodiiied alloy without the aging treatment. The most dramaticimprovement is shown with aged alloys containing zirconium. In that casethe endurance limit was raised to 19,000 p.s.i., representing a 46percent increase over the endurance limit of the as-cast 214X alloy. S/Ncurves (permissible stress/number of cycles before failure) are shown inFIG. 3.

As used in the specification the term consisting essentially of refersto essential elements of the alloy-to elements which are deliberatelymixed to form the desired alloy having a desired combination ofproperties. It should, however, be understood that small amounts ofother elements may be part of the alloy as claimed which are notdeliberately added but which appear in the nal alloy in the process ofits manufacture. Thus, such impurities as iron up to about 1.8 percent,copper up to 0.12v percent, and chromium up to 0.1 percent may betolerated without adversely influencing the desirable qualities thealloy. Hence, the presence of such impurities in the alloy as anon-preferred condition is deemed to be within the scope of the claims.

What is claimed is:

1. A castable, age-hardenable, aluminum-base alloy consistingessentially of, in weight percent, 3.0 to 4.5 magnesium, 0.6 to 1.8silicon, 0.4 to 1.5 silver, zirconium at a concentration in the range of0.2 to 0.5, and the balance aluminum.

2. As a new article of manufacture, a compressor blade casting of analloy consisting essentially of, in Weight percent, 3.0 to 4.5magnesium, 0.6 to 1.8 silicon, 0.4 to 1.5 silver, zirconium at aconcentration in the range of 0.2 to 0.5, and the balance aluminum, saidalloy being in an age-hardened condition by heat treatment at atemperature above the intended service temperature.

References Cited UNITED STATES PATENTS 3,306,717 2/1967 Lindstrand etal. l5-138 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R.

